There is a gaining enthusiasm for smaller and thinner (flat) imaging devices based on replacing hot cathode ray tube electron sources used in video tubes and X-ray imaging devices with field emission type electron sources. Examples of image capture devices using field emission type electron sources are visible light image capture devices as shown in, e.g., Japanese laid open publication JP 2000-48743A (the '743 publication) and X-ray image capture devices as shown in, e.g., Japanese laid open publication 2009-272289 (the '289 publication).
Video tubes using hot cathode electron sources, such as those shown in, e.g., Japanese laid-open publication JP H07-29507A (the '507 publication) as well as the above-mentioned prior art imaging devices comprising field emission type electron sources have typically made use of a grid electrode, e.g., a thin material with an array of small openings and having a grid-, mesh- or sieve-like structure, positioned between the anode and cathode. This grid electrode may also be referred to as a control grid or a trimming electrode. The grid electrode is typically for accelerating electrons from a hot cathode or a field emission type electron source and project the electron beam. The grid electrode may also improve the aim of electron beams by only allowing the passage of electron beams traveling orthogonally from the electron source and blocking electron beams having an angular component.
Reference is now made to FIG. 1, which shows a conventional, PRIOR ART image capture device with a field emission type electron source 15 and a grid electrode 20′, as shown in the '743 publication. The grid electrode 20′, positioned between the electron emitting construct (comprising the field emission type electron source 15) and the electron receiving construct (comprising the faceplate 3), accelerates and directs the electron beams from the field emission type electron sources 15 to a predetermined target area on the electron receiving construct.
Imaging devices comprising a grid electrode have the disadvantage of having a reduced utilization efficiency of the electron beams being emitted from the electron source. For example, when a grid electrode, e.g., as illustrated in the '507 publication, is used, electrons that fail to pass through the open area are absorbed into the grid and are lost without providing signal current. On the other hand, if the size of the grid electrode openings is widened (to increase utilization efficiency of the electron beams), another problem arises wherein electrons with an angular (i.e., non-perpendicular) component will pass through and hit the photoconductor outside of the predetermined target location. As such, electron beams may hit an adjacent pixel causing a readout in a pixel that is different from the target pixel, thus reducing image quality (e.g., resolution). In addition, the physical strength of the grid electrode become weaker as the aperture of the grid openings becomes wider. Therefore, it is difficult to assemble and maintain a grid with a large aperture. For at least these reasons, the ability to mitigate the reduced utilization efficiency of the electron beam caused by the grid electrode, by modifying the grid electrode, is limited.
Further, the grid electrode can become a source of microphonic noise in applications where the system must be moved during irradiation such as video imaging, CT scanning or Fluoroscopy. The interaction between the electron beam and the grid can create an energy spread in the electron beam, thus changing the system characteristics.
Finally, the presence of a grid electrode presents an assembly problem regardless of the grid opening aperture. This assembly problem is exacerbated in a large, thin imaging device such as a flat panel-type image capture device, in which the grid electrode must be assembled within a narrow gap in a precise manner, leading to increased defective products and increased cost of production.
The disclosure below addresses the above-described problems associated with conventional imaging devices using field emission type electron sources.